1. Field of the Invention
The present invention relates to a detector for detecting vibrations. More particularly the invention pertains to multi-dimensional or seismic vibration detectors.
2. Discussion of Related Art
There is a need in many contexts for a detector for detecting vibrations in, for example, a body to which the detector is fixed. Such a detector is suitable for employment in, for instance, the monitoring of bearings in rotary machines, for activating airbags, in burglar alarms etc. In many practical applications, it is also required that the direction of movement of the detected movement is identified.
For a detector which is used in a security system (burglar alarm), it is of great importance that the detector makes it possible at a very early stage to discover that a break-in is being attempted. It is highly desirable that the security system emits an alarm early, in other words before the break-in attempt has caused any major damage. This early warning system should be combined with an operative facility which ensures that an alarm is signalled only when a real attempted break-in is in progress.
It is previously known in the art to employ, in security systems, detectors which break or make a current circuit when, for example, a door is opened. The drawback in such detectors is that the alarm is made only when the thief has already gained access to the premises and, consequently, the thief has already caused material damage as a result of the break-in itself. The risk is imminent that the thief causes further damage to the interior of the premises when the alarm is sounded and/or when the thief takes goods away in connection with fleeing the scene of the crime.
Movement detectors in premises suffer from corresponding drawbacks since they only indicate a break-in once the thief is inside the premises.
Detectors based on sound analysis suffer from the drawback that the risk of a false alarm is great. In purely practical terms, it has proved that "break-in sound" cannot always be distinguished from "authorised" sound, for example in staircases in apartment buildings.